Gas chromatography has been known for many years and is used principally as an analytical technique for the determination of volatile compounds (gases and liquids) with boiling points up to approximately 500.degree. C. It is a separation technique involving the passage of a gaseous moving phase through a column containing a fixed phase. The separations are achieved by selective retardation exerted by the fixed phase because of differences in partition coefficients for gas-liquid chromatography. Of the three ways in which separation may be achieved, elution development is most commonly employed. Elution development requires the use only of a small sample which is injected at the inlet of a column. This sample is eluted by a constant flowing stream of a carrier gas which is generally inert (non-soluble and non-adsorbed), such as helium, nitrogen, or hydrogen. Because of the separation, the constituents of the sample substance appear as bands which travel through the column each at its own specific rate under the conditions of the test. Where the bands are separated, pure carrier gas emerges from the column between the different bands. Separation of the constituents or components of the sample is a first step in the analysis. The next step is detection of the presence of the constituents and their measurement.
Two principal methods are available for detecting and measuring the components in the effluent gas. These are classified as integral or differential types depending upon whether the effect produced on the device is measured additively or instantaneously. Differential detection is most widely used and techniques have been developed for analysis of thermal conductivity, gas density differences, and ionization properties. The most sensitive detectors are of the ionization type. Typically, the detector measures some physical property of the gases eluting from the column. When the composition of the gas eluting from the column changes, indicating that some sample component is eluting, the physical property being measured changes and this is sensed by the detector.
The operation of ionization detectors often is based upon the principle that a change in electrical conductance of an effluent gas is brought about by ionization of the sample component. One way in which ionization may be accomplished is to use a hydrogen flame ionization detector in which the ions are produced as the component burns. Another detector, the metastable ionization detector (MID) utilizes the Penning Effect of rare gases to detect sample components. Carrier gas molecules, usually helium, are used as the carrier, and these molecules are exposed to an electric field as well as to beta radiation from a radioactive source. This combination raises the carrier gas molecules to a metastable state with an ionization potential for helium of 19.8EV. As a result, all constituents having a lower ionization potential will be ionized giving rise to a positive signal output. The resulting electrons and ions are collected by the detector electrodes and typically measured by an electrometer coupled thereto. The metastable ionization detector (MID) provides an effective means by which the constituents of a sample may be detected, but it has drawbacks.
Although the MID is a very sensitive detector, it has a limited response range, i.e., at a given voltage setting, it can only quantitatively respond to sample concentrations over a limited range. At maximum sensitivity, the MID will detect constituents of a sample in the parts per billion (ppb) range, but will saturate for concentrations in the order of 10 parts per million (ppm). Saturation occurs when so much of a sample is ionized within the MID that the detector is overloaded and compensates for this overload by an arc discharge. Because arcing within the MID can damage the radioactive foil component of the detector, as well as create pits and holes in the MID walls and electrodes, care must be exercised to avoid the presence of such high concentrations. One technique by which arcing may be suppressed is to install a limiting resistor in the high voltage lead to one of the electrodes. By so doing, no meaningful quantitative data can be obtained when the detector is saturated.
One approach to a solution to this problem is disclosed in U.S. Pat. No. 4,345,154, issued Aug. 17, 1982 by Agustus S. Bainbridge, entitled "BIAS-COMPENSATED, IONIZATION SENSOR FOR GASEOUS MEDIA AND METHOD FOR ATTAINING PROPER BIAS FOR SAME", which discloses a technique for determining the most effective bias voltage and pre-selected resistance to be provided by the biasing means. Thus, the patent discloses a method for biasing an ionization sensor so as to minimize the effect of the gas flow rate change on the output signal from the detector. The bias voltage is empirically determined by varying the flow rate and observing the output signal changes for different values of bias voltage. Once the value of the optimum bias voltage has been determined, this value is then applied as a fixed bias to the detector. In contrast, the present invention discloses a technique for significantly extending the dynamic range of a metastable ionization detector by automatically and continuously varying the gain of the detector as a function of its own output signal.